Polyelectrolyte Multilayers: Towards Single Cell Studies

Mechanobiology of Ferroptotic Cancer Cells as a Novel "Eat-Me" Signal: Regulating Efferocytosis through Layer-by-Layer Coating

The importance of the clearance of dead cells is shown to have a regulatory role for normal tissue homeostasis and for the modulation of immune responses. However, how mechanobiological properties of dead cells affect efferocytosis remains largely unknown. Here, it is reported that the Young's modulus of cancer cells undergoing ferroptosis is reduced. To modulate their Young's modulus a layer-by-layer (LbL) nanocoating is developed. Scanning electron and fluorescence microscopy confirm coating efficiency of ferroptotic cells while atomic force microscopy reveals encapsulation of the dead cells increases their Young's modulus dependent on the number of applied LbL layers which increases their efferocytosis by primary macrophages. This work demonstrates the crucial role of mechanobiology of dead cells in regulating their efferocytosis by macrophages which can be exploited for the development of novel therapeutic strategies for diseases where modulation of efferocytosis can be potentially beneficial and for the design of drug delivery systems for cancer therapy.

Spontaneous shrinkage drives macromolecule encapsulation into layer-by-layer assembled biopolymer microgels

HYPOTHESIS

Recently, the anomalous shrinkage of surface-supported hyaluronate/poly-l-lysine (HA/PLL) microgels (µ-gels), which exceeds that reported for any other multilayer-based systems, has been reported [1]. The current study investigates the capability of these unique µ-gels for the encapsulation and retention of macromolecules, and proposes the shrinkage-driven assembly of biopolymer-based µ-gels as a novel tool for one-step surface biofunctionalization.

EXPERIMENTS

A set of dextrans (DEX) and their charged derivatives - carboxymethyl (CM)-DEX and diethylaminoethyl (DEAE)-DEX - has been utilized to evaluate the effects of macromolecular mass and net charge on µ-gel shrinkage and macromolecule entrapment. µ-gels formation on the surface of silicone catheters exemplifies their potential to tailor biointerfaces.

FINDINGS

Shrinkage-driven µ-gel formation strongly depends on the net charge and mass content of encapsulated macromolecules. Inclusion of neutral DEX decreases the degree of shrinkage several times, whilst charged DEXs adopt to the backbone of oppositely charged polyelectrolytes, resulting in shrinkage comparable to that of non-loaded µ-gels. Retention of CM-DEX in µ-gels is significantly higher compared to DEAE-DEX. These insights into the mechanisms of macromolecular entrapment into biopolymer-based µ-gels promotes fundamental understanding of molecular dynamics within the multilayer assemblies. Organization of biodegradable µ-gels at biomaterial surfaces opens avenues for their further exploitation in a diverse array of bioapplications.

A lipid membrane supported on an artificial extracellular matrix made of polyelectrolyte multilayers: towards nanoarchitectonics at the cellular interface

To implement a specific function, cells recognize multiple physical and chemical cues and exhibit molecular responses at their interfaces - the boundary regions between the cell lipid-based membrane and the surrounding extracellular matrix (ECM). Mimicking the cellular external microenvironment presents a big challenge in nanoarchitectonics due to the complexity of the ECM and lipid membrane fragility. This study reports an approach for the assembly of a lipid bilayer, mimicking the cellular membrane, placed on top of a polyelectrolyte multilayer cushion made of hyaluronic acid and poly-L-lysine - a nanostructured biomaterial, which represents a 3D artificial ECM. Model proteins, lysozyme and α-lactalbumin, (which have similar molecular masses but carry opposite net charges) have been employed as soluble signalling molecules to probe their interaction with these hybrids. The formation of a lipid bilayer and the intermolecular interactions in the hybrid structure are monitored using a quartz crystal microbalance and confocal fluorescence microscopy. Electrostatic interactions between poly-L-lysine and the externally added proteins govern the transport of proteins into the hybrid. Designed ECM-cell mimicking hybrids open up new avenues for modelling a broad range of cell membranes and ECM and their associated phenomena, which can be used as a tool for synthetic biology and drug screening.

Modification of Surfaces with Vaterite CaCO3 Particles

Former studies have demonstrated a strong interest toward the crystallization of CaCO3 polymorphs in solution. Nowadays, CaCO3 crystallization on solid surfaces is extensively being studied using biomolecules as substrates for the control of the growth aiming at various applications of CaCO3. Calcium carbonate exists in an amorphous state, as three anhydrous polymorphs (aragonite, calcite and vaterite), and as two hydrated polymorphs (monohydrocalcite and ikaite). The vaterite polymorph is considered as one of the most attractive forms due to its large surface area, biocompatibility, mesoporous nature, and other features. Based on physical or chemical immobilization approaches, vaterite can be grown directly on solid surfaces using various (bio)molecules, including synthetic polymers, biomacromolecules such as proteins and peptides, carbohydrates, fibers, extracellular matrix components, and even biological cells such as bacteria. Herein, the progress on the modification of solid surfaces by vaterite CaCO3 crystals is reviewed, focusing on main findings and the mechanism of vaterite growth initiated by various substances mentioned above, as well as the discussion of the applications of such modified surfaces.

Biopolymer-Based Multilayer Capsules and Beads Made via Templating: Advantages, Hurdles and Perspectives

One of the undeniable trends in modern bioengineering and nanotechnology is the use of various biomolecules, primarily of a polymeric nature, for the design and formulation of novel functional materials for controlled and targeted drug delivery, bioimaging and theranostics, tissue engineering, and other bioapplications. Biocompatibility, biodegradability, the possibility of replicating natural cellular microenvironments, and the minimal toxicity typical of biogenic polymers are features that have secured a growing interest in them as the building blocks for biomaterials of the fourth generation. Many recent studies showed the promise of the hard-templating approach for the fabrication of nano- and microparticles utilizing biopolymers. This review covers these studies, bringing together up-to-date knowledge on biopolymer-based multilayer capsules and beads, critically assessing the progress made in this field of research, and outlining the current challenges and perspectives of these architectures. According to the classification of the templates, the review sequentially considers biopolymer structures templated on non-porous particles, porous particles, and crystal drugs. Opportunities for the functionalization of biopolymer-based capsules to tailor them toward specific bioapplications is highlighted in a separate section.

Which Biopolymers Are Better for the Fabrication of Multilayer Capsules? A Comparative Study Using Vaterite CaCO3 as Templates

The polymer layer-by-layer assembly is accounted among the most attractive approaches for the design of advanced drug delivery platforms and biomimetic materials in 2D and 3D. The multilayer capsules can be made of synthetic or biologically relevant (e.g., natural) polymers. The biopolymers are advantageous for bioapplications; however, the design of such "biocapsules" is more challengeable due to intrinsic complexity and lability of biopolymers. Until now, there are no systematic studies that report the formation mechanism for multilayer biocapsules templated upon CaCO₃ crystals. This work evaluates the structure-property relationship for 16 types of capsules made of different biopolymers and proposes the capsule formation mechanism. The capsules have been fabricated upon mesoporous cores of vaterite CaCO₃, which served as a sacrificial template. Stable capsules of polycations poly-l-lysine or protamine and four different polyanions were successfully formed. However, capsules made using the polycation collagen and dextran amine underwent dissolution. Formation of the capsules has been correlated with the stability of the respective polyelectrolyte complexes at increased ionic strength. All formed capsules shrink upon core dissolution and the degree of shrinkage increased in the series of polyanions: heparin sulfate < dextran sulfate < chondroitin sulfate < hyaluronic acid. The same trend is observed for capsule adhesiveness to the glass surface, which correlates with the decrease in polymer charge density. The biopolymer length and charge density govern the capsule stability and internal structure; all formed biocapsules are of a matrix-type, other words are microgels. These findings can be translated to other biopolymers to predict biocapsule properties.

CaCO3 crystals as versatile carriers for controlled delivery of antimicrobials

CaCO₃ crystals have been known for a long time as naturally derived and simply fabricated nano(micro)-sized materials able to effectively host and release various molecules. This review summarises the use of CaCO₃ crystals as versatile carriers to host, protect and release antimicrobials, offering a strong tool to tackle antimicrobial resistance, a serious global health problem. The main methods for the synthesis of CaCO₃ crystals with different properties, as well as the approaches for the loading and release of antimicrobials are presented. Finally, prospects to utilize the crystals in order to improve the therapeutic outcome and combat antimicrobial resistance are highlighted. Ultimately, this review intends to provide an in-depth overview of the application of CaCO₃ crystals for the smart and controlled delivery of antimicrobial agents and aims at identifying the advantages and drawbacks as well as guiding future works, research directions and industrial applications.

Layer-By-Layer Assemblies of Biopolymers: Build-Up, Mechanical Stability and Molecular Dynamics

Rapid development of versatile layer-by-layer technology has resulted in important breakthroughs in the understanding of the nature of molecular interactions in multilayer assemblies made of polyelectrolytes. Nowadays, polyelectrolyte multilayers (PEM) are considered to be non-equilibrium and highly dynamic structures. High interest in biomedical applications of PEMs has attracted attention to PEMs made of biopolymers. Recent studies suggest that biopolymer dynamics determines the fate and the properties of such PEMs; however, deciphering, predicting and controlling the dynamics of polymers remains a challenge. This review brings together the up-to-date knowledge of the role of molecular dynamics in multilayers assembled from biopolymers. We discuss how molecular dynamics determines the properties of these PEMs from the nano to the macro scale, focusing on its role in PEM formation and non-enzymatic degradation. We summarize the factors allowing the control of molecular dynamics within PEMs, and therefore to tailor polymer multilayers on demand.

Polyelectrolyte Multilayer Capsule (PEMC)-Based Scaffolds for Tissue Engineering

Tissue engineering (TE) is a highly multidisciplinary field that focuses on novel regenerative treatments and seeks to tackle problems relating to tissue growth both in vitro and in vivo. These issues currently involve the replacement and regeneration of defective tissues, as well as drug testing and other related bioapplications. The key approach in TE is to employ artificial structures (scaffolds) to support tissue development; these constructs should be capable of hosting, protecting and releasing bioactives that guide cellular behaviour. A straightforward approach to integrating bioactives into the scaffolds is discussed utilising polyelectrolyte multilayer capsules (PEMCs). Herein, this review illustrates the recent progress in the use of CaCO3 vaterite-templated PEMCs for the fabrication of functional scaffolds for TE applications, including bone TE as one of the main targets of PEMCs. Approaches for PEMC integration into scaffolds is addressed, taking into account the formulation, advantages, and disadvantages of such PEMCs, together with future perspectives of such architectures.

Adsorption and Release of Rose Bengal on Layer-by-Layer Films of Poly(Vinyl Alcohol) and Poly(Amidoamine) Dendrimers Bearing 4-Carboxyphenylboronic Acid

Phenylboronic acid-bearing polyamidoamine dendrimer (PBA-PAMAM)/poly(vinyl alcohol) (PVA) multilayer films were prepared through the layer-by-layer (LbL) deposition of PBA-PAMAM solution and PVA solution. PBA-PAMAM/PVA films were constructed successfully through the formation of boronate ester bonds between the boronic acid moiety in PBA and 1,3-diol units in PVA. When the (PBA-PAMAM/PVA)₅ films were immersed in rose bengal (RB) solution, RB was adsorbed onto the LbL films. The amount of RB adsorbed was higher in the LbL films immersed in acidic solution than in basic solution. The release of RB from the LbL films was also promoted in the basic solution, while it was suppressed in the acidic solution. The boronic acid ester is oxidized to phenol by hydrogen peroxide (H₂O₂) and the carbon-boron bond is cleaved, so that the (PBA-PAMAM/PVA)₅ films can be decomposed by immersion in H₂O₂ solution. Therefore, when RB-adsorbed (PBA-PAMAM/PVA)₅ films were immersed in H₂O₂ solution, the release of RB was moderately promoted when the solution was weakly acidic.

Encapsulation of Low-Molecular-Weight Drugs into Polymer Multilayer Capsules Templated on Vaterite CaCO3 Crystals

Polyelectrolyte multilayer capsules (PEMCs) templated onto biocompatible and easily degradable vaterite CaCO₃ crystals via the layer-by-layer (LbL) polymer deposition process have served as multifunctional and tailor-made vehicles for advanced drug delivery. Since the last two decades, the PEMCs were utilized for effective encapsulation and controlled release of bioactive macromolecules (proteins, nucleic acids, etc.). However, their capacity to host low-molecular-weight (LMW) drugs (<1–2 kDa) has been demonstrated rather recently due to a limited retention ability of multilayers to small molecules. The safe and controlled delivery of LMW drugs plays a vital role for the treatment of cancers and other diseases, and, due to their tunable and inherent properties, PEMCs have shown to be good candidates for smart drug delivery. Herein, we summarize recent progress on the encapsulation of LMW drugs into PEMCs templated onto vaterite CaCO₃ crystals. The drug loading and release mechanisms, advantages and limitations of the PEMCs as LMW drug carriers, as well as bio-applications of drug-laden capsules are discussed based upon the recent literature findings.

Stationary photocurrent generation from bacteriorhodopsin-loaded lipo-polymersomes in polyelectrolyte multilayer assembly on polyethersulfone membrane

Vesicles constructed of either synthetic polymers alone (polymersomes) or a combination of polymers and lipids (lipo-polymersomes) demonstrate excellent long-term stability and ability to integrate membrane proteins. Applications using lipo-polymersomes with integrated membrane proteins require suitable supports to maintain protein functionality. Using lipo-polymersomes loaded with the light-driven proton pump bacteriorhodopsin (BR), we demonstrate here how the photocurrent is influenced by a chosen support. In our study, we deposited BR-loaded lipo-polymersomes in a cross-linked polyelectrolyte multilayer assembly either directly physisorbed on gold electrode microchips or cross-linked on an intermediary polyethersulfone (PES) membrane covalently grafted using a hydrogel cushion. In both cases, electrochemical impedance spectroscopic characterization demonstrated successful polyelectrolyte assembly with BR-loaded lipo-polymersomes. Light-induced proton pumping by BR-loaded lipo-polymersomes in the different support constructs was characterized by amperometric recording of the generated photocurrent. Application of the hydrogel/PES membrane support together with the polyelectrolyte assembly decreased the transient current response upon light activation of BR, while enhancing the generated stationary current to over 700 nA/cm². On the other hand, the current response from BR-loaded lipo-polymersomes in a polyelectrolyte assembly without the hydrogel/PES membrane support was primarily a transient peak combined with a low-nanoampere-level stationary photocurrent. Hence, the obtained results demonstrated that by using a hydrogel/PES support it was feasible to monitor continuously light-induced proton flux in biomimetic applications of lipo-polymersomes. Graphical abstract.

Atomistic simulation of PDADMAC/PSS oligoelectrolyte multilayers: overall comparison of tri- and tetra-layer systems

By employing large-scale molecular dynamics simulations of atomistically resolved oligoelectrolytes in aqueous solutions, we study in detail the first four layer-by-layer deposition cycles of an oligoelectrolyte multilayer made of poly(diallyl dimethyl ammonium chloride)/poly(styrene sulfonate sodium salt) (PDADMAC/PSS). The multilayers are grown on a silica substrate in 0.1 M NaCl electrolyte solutions and the swollen structures are then subsequently exposed to varying added salt concentration. We investigated the microscopic properties of the films, analyzing in detail the differences between three- and four-layer systems. Our simulations provide insights into the early stages of growth of a multilayer, which are particularly challenging for experimental observations. We found rather strong complexation of the oligoelectrolytes, with fuzzy layering of the film structure. The main charge compensation mechanism is for all cases intrinsic, whereas extrinsic compensation is relatively enhanced for the layer of the last deposition cycle. In addition, we quantified other fundamental observables of these systems, such as the film thickness, water uptake, and overcharge fractions for each deposition layer.

Hybrids of Polymer Multilayers, Lipids, and Nanoparticles: Mimicking the Cellular Microenvironment

Here we address research directions and trends developed following novel concepts in 2D/3D self-assembled polymer structures established in the department led by Helmuth Möhwald. These functional structures made of hybrids of polymer multilayers, lipids, and nanoparticles stimulated research in the design of the cellular microenvironment. The composition of the extracellular matrix (ECM) and dynamics of biofactor presentation in the ECM can be recapitulated by the hybrids. Proteins serve as models for protein-based biofactors such as growth factors, cytokines, hormones, and so forth. A fundamental understanding of complex intermolecular interactions and approaches developed for the externally IR-light-triggered release offers a powerful tool for controlling the biofactor presentation. Pure protein beads made via a mild templating on vaterite CaCO₃ crystals can mimic cellular organelles in terms of the compartmentalization of active proteins. We believe that an integration of the approaches developed and described here offers a strong tool for engineering and mimicking both extra- and intracellular microenvironments.

The Future of Layer-by-Layer Assembly: A Tribute to ACS Nano Associate Editor Helmuth Möhwald

Layer-by-layer (LbL) assembly is a widely used tool for engineering materials and coatings. In this Perspective, dedicated to the memory of ACS Nano associate editor Prof. Dr. Helmuth Möhwald, we discuss the developments and applications that are to come in LbL assembly, focusing on coatings, bulk materials, membranes, nanocomposites, and delivery vehicles.

Porous Alginate Scaffolds Assembled Using Vaterite CaCO3 Crystals

Formulation of multifunctional biopolymer-based scaffolds is one of the major focuses in modern tissue engineering and regenerative medicine. Besides proper mechanical/chemical properties, an ideal scaffold should: (i) possess a well-tuned porous internal structure for cell seeding/growth and (ii) host bioactive molecules to be protected against biodegradation and presented to cells when required. Alginate hydrogels were extensively developed to serve as scaffolds, and recent advances in the hydrogel formulation demonstrate their applicability as "ideal" soft scaffolds. This review focuses on advanced porous alginate scaffolds (PAS) fabricated using hard templating on vaterite CaCO3 crystals. These novel tailor-made soft structures can be prepared at physiologically relevant conditions offering a high level of control over their internal structure and high performance for loading/release of bioactive macromolecules. The novel approach to assemble PAS is compared with traditional methods used for fabrication of porous alginate hydrogels. Finally, future perspectives and applications of PAS for advanced cell culture, tissue engineering, and drug testing are discussed.

Internal Structure of Matrix-Type Multilayer Capsules Templated on Porous Vaterite CaCO₃ Crystals as Probed by Staining with a Fluorescence Dye

Multilayer capsules templated on decomposable vaterite CaCO₃ crystals are widely used as vehicles for drug delivery. The capsule represents typically not a hollow but matrix-like structure due to polymer diffusion into the porous crystals during multilayer deposition. The capsule formation mechanism is not well-studied but its understanding is crucial to tune capsule structure for a proper drug release performance. This study proposes new approach to noninvasively probe and adjust internal capsule structure. Polymer capsules made of poly(styrene-sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDAD) have been stained with fluorescence dye rhodamine 6G. Physical-chemical aspects of intermolecular interactions required to validate the approach and adjust capsule structure are addressed. The capsules consist of a defined shell (typically 0.5⁻2 µm) and an internal matrix of PSS-PDAD complex (typically 10⁻40% of a total capsule volume). An increase of ionic strength and polymer deposition time leads to the thickening of the capsule shell and formation of a denser internal matrix, respectively. This is explained by effects of a polymer conformation and limitations in polymer diffusion through the crystal pores. We believe that the design of the capsules with desired internal structure will allow achieving effective encapsulation and controlled/programmed release of bioactives for advanced drug delivery applications.

On the Infectivity of Bacteriophages in Polyelectrolyte Multilayer Films: Inhibition or Preservation of Their Bacteriolytic Activity?

Antibiotic resistance in bacterial cells has motivated the scientific community to design new and efficient (bio)materials with targeted bacteriostatic and/or bactericide properties. In this work, a series of polyelectrolyte multilayer films differing in terms of polycation-polyanion combinations are constructed according to the layer-by-layer deposition method. Their capacities to host T4 and φx174 phage particles and maintain their infectivity and bacteriolytic activity are thoroughly examined. It is found that the macroscopic physicochemical properties of the films, which includes film thickness, swelling ratio, or mechanical stiffness (as derived by atomic force microscopy and spectroscopy measurements), do not predominantly control the selectivity of the films for hosting infective phages. Instead, it is evidenced that the intimate electrostatic interactions locally operational between the loaded phages and the polycationic and polyanionic PEM components may lead to phage activity reduction and preservation/enhancement, respectively. It is argued that the underlying mechanism involves the screening of the phage capsid receptors (operational in cell recognition/infection processes) because of the formation of either polymer-phage hetero-assemblies or polymer coating surrounding the bioactive phage surface.

Preparation of Nafion/Polycation Layer-by-Layer Films for Adsorption and Release of Insulin

Thin films were prepared using layer-by-layer (LbL) deposition of Nafion (NAF) and polycations such as poly(allylamine hydrochloride) (PAH), poly(ethyleneimine) (PEI), and poly(diallydimethylammonium chloride) (PDDA). Insulin was then adsorbed on the NAF-polycation LbL films by immersion in an insulin solution. The NAF-polycation LbL films were characterized using a quartz crystal microbalance and an atomic force microscope. The release of insulin from the LbL films was characterized using UV-visible adsorption spectroscopy and fluorescence emission spectroscopy. The greatest amount of insulin was adsorbed on the NAF-PAH LbL film. The amount of insulin adsorbed on the (NAF/PAH)₅NAF LbL films by immersion in a 1 mg mL-1 insulin solution at pH 7.4 was 61.8 µg cm-2. The amount of insulin released from the LbL films was higher when immersed in insulin solutions at pH 2.0 and pH 9.0 than at pH 7.4. Therefore, NAF-polycations could be employed as insulin delivery LbL films under mild conditions and as an insulin release control system according to pH change.

Binding Mechanism of the Model Charged Dye Carboxyfluorescein to Hyaluronan/Polylysine Multilayers

Biopolymer-based multilayers become more and more attractive due to the vast span of biological application they can be used for, e.g., implant coatings, cell culture supports, scaffolds. Multilayers have demonstrated superior capability to store enormous amounts of small charged molecules, such as drugs, and release them in a controlled manner; however, the binding mechanism for drug loading into the multilayers is still poorly understood. Here we focus on this mechanism using model hyaluronan/polylysine (HA/PLL) multilayers and a model charged dye, carboxyfluorescein (CF). We found that CF reaches a concentration of 13 mM in the multilayers that by far exceeds its solubility in water. The high loading is not related to the aggregation of CF in the multilayers. In the multilayers, CF molecules bind to free amino groups of PLL; however, intermolecular CF-CF interactions also play a role and (i) endow the binding with a cooperative nature and (ii) result in polyadsorption of CF molecules, as proven by fitting of the adsorption isotherm using the BET model. Analysis of CF mobility in the multilayers by fluorescence recovery after photobleaching has revealed that CF diffusion in the multilayers is likely a result of both jumping of CF molecules from one amino group to another and movement, together with a PLL chain being bound to it. We believe that this study may help in the design of tailor-made multilayers that act as advanced drug delivery platforms for a variety of bioapplications where high loading and controlled release are strongly desired.

Sugar-sensitive dendrimer films as a sacrificial layer for the preparation of freestanding multilayer films

Multilayer thin films composed of poly(vinyl alcohol) (PVA) and phenylboronic acid-bearing poly(amidoamine) dendrimer (PBA-PAMAM) were used as a sacrificial layer for constructing freestanding polyelectrolyte films consisting of poly(styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH). Freestanding (PSS/PAH)₁₅ films were successfully released from substrate by exposing composite (PVA/PBA-PAMAM)_n/(PSS/PAH)₁₅ films (n=5 and 10) to sorbitol solutions under mild conditions at pH7.0-9.0. The film release was accelerated in solutions of higher sorbitol concentrations at pH9.0 as well as in solutions with lower concentration of NaCl. The results were rationalized based on the scission of boronate ester bonds between PBA-PAMAM and PVA in the (PVA/PBA-PAMAM)_n layer due to a competitive binding of sorbitol to PBA-PAMAM.

Hydrophilic Polyelectrolyte Multilayers Improve the ELISA System: Antibody Enrichment and Blocking Free

In this study, polyelectrolyte multilayers were fabricated on a polystyrene (PS) plate using a Layer-by-Layer (LbL) self-assembly technique. The resulting functional platform showed improved performance compared with conventional enzyme-linked immunosorbent assay (ELISA) systems. Poly(diallyldimethylammonium chloride) (PDDA) and poly(acrylic acid) (PAA) were used as cationic and anionic polyelectrolytes. On the negatively-charged (PDDA/PAA)₃ polyelectrolyte multilayers the hydrophilic PAA surface could efficiently decrease the magnitude of the noise signal, by inhibiting nonspecific adsorption even without blocking reagent adsorption. Moreover, the (PDDA/PAA)₃ substrate covalently immobilized the primary antibody, greatly increasing the amount of primary antibody adsorption and enhancing the specific detection signal compared with a conventional PS plate. The calibration curve of the (PDDA/PAA)₃ substrate showed a wide linear range, for concentrations from 0.033 to 33 nM, a large specific signal change, and a detection limit of 33 pM, even though the conventional blocking reagent adsorption step was omitted. The (PDDA/PAA)₃ substrate provided a high-performance ELISA system with a simple fabrication process and high sensitivity; the system presented here shows potential for a variety of immunosensor applications.

Temperature effect on the build-up of exponentially growing polyelectrolyte multilayers. An exponential-to-linear transition point

In this study, the effect of temperature on the build-up of exponentially growing polyelectrolyte multilayer films was investigated. It aims at understanding the multilayer growth mechanism as crucially important for the fabrication of tailor-made multilayer films. Model poly(L-lysine)/hyaluronic acid (PLL/HA) multilayers were assembled in the temperature range of 25-85 °C by layer-by-layer deposition using a dipping method. The film growth switches from the exponential to the linear regime at the transition point as a result of limited polymer diffusion into the film. With the increase of the build-up temperature the film growth rate is enhanced in both regimes; the position of the transition point shifts to a higher number of deposition steps confirming the diffusion-mediated growth mechanism. Not only the faster polymer diffusion into the film but also more porous/permeable film structure are responsible for faster film growth at higher preparation temperature. The latter mechanism is assumed from analysis of the film growth rate upon switching of the preparation temperature during the film growth. Interestingly, the as-prepared films are equilibrated and remain intact (no swelling or shrinking) during temperature variation in the range of 25-45 °C. The average activation energy for complexation between PLL and HA in the multilayers calculated from the Arrhenius plot has been found to be about 0.3 kJ mol(-1) for monomers of PLL. Finally, the following processes known to be dependent on temperature are discussed with respect to the multilayer growth: (i) polymer diffusion, (ii) polymer conformational changes, and (iii) inter-polymer interactions.

Preparation of Layer-by-Layer Films Composed of Polysaccharides and Poly(Amidoamine) Dendrimer Bearing Phenylboronic Acid and Their pH- and Sugar-Dependent Stability

Layer-by-layer films composed of polysaccharides and poly(amidoamine) dendrimer bearing phenylboronic acid (PBA-PAMAM) were prepared to study the deposition behavior of the films and their stability in buffer solutions and in sugar solutions. Alginic acid (AGA) and carboxymethylcellulose (CMC) were employed as counter-polymers in constructing LbL films. AGA/PBA-PAMAM films were successfully prepared at pH 6.0-9.0, whereas the preparation of CMC/PBA-PAMAM film was unsuccessful at pH 8.0 and 9.0. The results show that the LbL films formed mainly through electrostatic affinity between PBA-PAMAM and polysaccharides, while, for AGA/PBA-PAMAM films, the participation of boronate ester bonds in the films was suggested. AGA/PBA-PAMAM films were stable in the solutions of pH 6.0-9.0. In contrast, CMC/PBA-PAMAM films decomposed at pH 7.5-9.0. The AGA/PBA-PAMAM films decomposed in response to 5-30 mM fructose at pH 7.5, while the films were stable in glucose solutions. Thus, AGA is useful as a counter-polymer for constructing PBA-PAMAM films that are stable at physiological pH and decompose in response to fructose.

Composite SERS-based satellites navigated by optical tweezers for single cell analysis

Herein, we have designed composite SERS-active micro-satellites, which exhibit a dual role: (i) effective probes for determining cellular composition and (ii) optically movable and easily detectable markers. The satellites were synthesized by the layer-by-layer assisted decoration of silica microparticles with metal (gold or silver) nanoparticles and astralen in order to ensure satellite SERS-based microenvironment probing and satellite recognition, respectively. A combination of optical tweezers and Raman spectroscopy can be used to navigate the satellites to a certain cellular compartment and probe the intracellular composition following cellular uptake. In the future, this developed approach may serve as a tool for single cell analysis with nanometer precision due to the multilayer surface design, focusing on both extracellular and intracellular studies.

Polyelectrolyte multilayer film modification for chemo-mechano-regulation of endothelial cell response

The new multilayer polyelectrolyte films (PEMs) that are able to simulate the structure and functions of the extracellular matrix have become a powerful tool for tailoring biointerfaces of “cardiovascular” implants.

Ionic Driven Embedment of Hyaluronic Acid Coated Liposomes in Polyelectrolyte Multilayer Films for Local Therapeutic Delivery

The ability to control the spatial distribution and temporal release of a therapeutic remains a central challenge for biomedical research. Here, we report the development and optimization of a novel substrate mediated therapeutic delivery system comprising of hyaluronic acid covalently functionalized liposomes (HALNPs) embedded into polyelectrolyte multilayer (PEM) platform via ionic stabilization. The PEM platform was constructed from sequential deposition of Poly-L-Lysine (PLL) and Poly(Sodium styrene sulfonate) (SPS) "(PLL/SPS)4.5" followed by adsorption of anionic HALNPs. An adsorption affinity assay and saturation curve illustrated the preferential HALNP deposition density for precise therapeutic loading. (PLL/SPS)2.5 capping layer on top of the deposited HALNP monolayer further facilitated complete nanoparticle immobilization, cell adhesion, and provided nanoparticle confinement for controlled linear release profiles of the nanocarrier and encapsulated cargo. To our knowledge, this is the first study to demonstrate the successful embedment of a translatable lipid based nanocarrier into a substrate that allows for temporal and spatial release of both hydrophobic and hydrophilic drugs. Specifically, we have utilized our platform to deliver chemotherapeutic drug Doxorubicin from PEM confined HALNPs. Overall, we believe the development of our HALNP embedded PEM system is significant and will catalyze the usage of substrate mediated delivery platforms in biomedical applications.

Spatio-Temporal Control of LbL Films for Biomedical Applications: From 2D to 3D

Introduced in the '90s by Prof. Moehwald, Lvov, and Decher, the layer-by-layer (LbL) assembly of polyelectrolytes has become a popular technique to engineer various types of objects such as films, capsules and free standing membranes, with an unprecedented control at the nanometer and micrometer scales. The LbL technique allows to engineer biofunctional surface coatings, which may be dedicated to biomedical applications in vivo but also to fundamental studies and diagnosis in vitro. Initially mostly developed as 2D coatings and hollow capsules, the range of complex objects created by the LbL technique has greatly expanded in the past 10 years. In this Review, the aim is to highlight the recent progress in the field of LbL films for biomedical applications and to discuss the various ways to spatially and temporally control the biochemical and mechanical properties of multilayers. In particular, three major developments of LbL films are discussed: 1) the new methods and templates to engineer LbL films and control cellular processes from adhesion to differentiation, 2) the major ways to achieve temporal control by chemical, biological and physical triggers and, 3) the combinations of LbL technique, cells and scaffolds for repairing 3D tissues, including cardio-vascular devices, bone implants and neuro-prosthetic devices.

Nanocapsules templated on liquid cores stabilized by graft amphiphilic polyelectrolytes

A surfactant-free method of preparation of nanocapsules templated on liquid cores using amphiphilic graft polyelectrolytes was developed. A model photoactive copolymer, poly(sodium 2-acrylamido-2-methyl-1-propanesulfonate) with grafted poly(vinylnaphthalene) chains (PAMPS-graft-PVN) was used to stabilize toluene droplets in an aqueous emulsion. The macromolecules, due to their amphiphilic character and the presence of strong ionic groups, tend to undergo intramolecular aggregation in water but at the water-oil interface less compact conformation is preferred with PVN grafts anchoring in the oil phase and the charged PAMPS main chains residing in the aqueous phase, thus stabilizing the nanoemulsion droplets. Formation of such nanocapsules was confirmed by dynamic light scattering measurements as well as SEM and cryo-TEM imaging. Grafting density and content of the chromophores in the graft copolymers were varied in order to achieve high stability of the coated nanodroplets. It was shown that the capsules are better stabilized by the copolymers with many short hydrophobic grafts than with fewer but longer ones. Use of photoactive polyelectrolytes enabled spectroscopic investigation of the relationship between conformation of the macromolecules and stabilization of the oil-core nanocapsules. Long-term stability of the nanocapsules was achieved and further increased by multilayer shell formation using polyelectrolytes deposited via the layer-by-layer approach. The obtained capsules served as efficient nanocontainers for a hydrophobic fluorescent probe. The proposed strategy of nanocapsule preparation may be easily extended to biologically relevant polymers and applied to fabricate liquid core nanodelivery systems without the need of using low molecular weight additives which may have adverse effects in numerous biomedical applications.